As devices shrink and levels of integration increase in today's rapidly emerging semiconductor manufacturing industry, it is critical to accurately monitor the components used to form semiconductor devices, including the materials used to form the components. This monitoring is especially critical for the conductive interconnect features used to couple the thousands of discrete component devices that combine to form an integrated circuit. High speed devices, in particular, require that the conductive interconnect materials used to couple the discrete components to one another and to bond pads for external communication, are high quality materials with high conductivities required for carrying high speed signals. Various metals have been used as conductive interconnects in semiconductor devices. Copper is a metal that is a favored conductive interconnect material for high speed devices because of its high conductivity and current carrying ability. It is therefore particularly critical to be able to evaluate the film quality of a copper interconnect film, particularly the resistivity and/or conductivity of the copper film which affects its current carrying ability. The conductivity of a copper film is known to be related to grain size of the film.
After a conductive material such as copper is initially formed as a film on a substrate to act as an interconnect material, the conductive material is annealed to improve various of its qualities including its current carrying speed. Annealing refers to a heat treatment wherein the microstructure of the material is altered causing changes in properties such as strength and hardness. Annealing typically results in softening of the conductive material through removal of crystal defects and the internal stresses which they cause. During the annealing process, the grain boundaries of the conductive material are typically reduced to minimum values, or saturated. Conversely, grain size is increased. This grain boundary variation influences conductivity which varies directly with grain size and conversely with hardness. Since the annealing process is intended to improve the quality of the copper film for its intended use, it would also be advantageous to monitor and evaluate the effectiveness of the annealing process, as well as the annealed copper film.
Conventional techniques for monitoring the film quality of a copper film and for determining its conductivity utilize a sheet resistance measurement, referred to as “sheet rho”, Rs. Annealing efficiency is conventionally monitored by comparing sheet rho measurements before and after annealing. A shortcoming of the sheet resistance measurement is that it estimates the film resistivity determine conductivity and therefore the speed of the copper interconnect, whereas the conductivity is actually more directly dependent upon the grain size of the copper film. The sheet resistance measurement, however, cannot measure grain size. In other words, the Rs measurement will be the same regardless of the saturation level of the copper grain boundaries or the copper grain sizes, because it is insensitive to differences in grain structure. Moreover, the sheet rho of the film is typically measured at one point using a 4-point probe and this localized measurement is therefore insensitive to variations in film resistivity throughout the film. Such a measurement technique necessarily relies on the assumption that the bulk resistivity is constant throughout the film, but in practice, it is not. Therefore, a sheet resistivity measurement is an inaccurate way to measure conductivity which depends on grain size and the conventional single, localized 4-point probe sheet resistivity measurement is also insensitive to variations in the bulk resistivity throughout the film which may be due to different grain structures in both annealed and unannealed films. In summary, this 4-point probe measurement of sheet resistivity is an inaccurate way of monitoring copper film quality, or the effectiveness or efficiency of the annealing process intended to improve conductivity and increase grain size.
It would therefore be desirable to provide a method for easily and accurately monitoring the conductivity of a film and the effectiveness of an annealing operation used to saturate grain boundaries of a film and improve its conductivity.